Facsimile signaling system



Jan. 30, 1945. F. P. MASON FACSIMILE SIGNALING SYSTEM 5 Sheets-Sheet 1 Filed Jan. 6, 1942 Jan.'30, 1945. MASON 2,368,369

FAC'SIMILE S IGNALING SYSTEM Filed Jan. 6, 1942 3 Sheets-Sheet 2 'WIVIIIVIIVAFI 6 j #wavrok 4 77' OR/VE Y Jan- 30, P. MASON FACSIMILE SIGNALING SYSTEM Filed Jan. 6, 1942 3 Sheets-Sheet 5 Patented Jan. 30, 1945 FACSHHILE SIGNALING SYSTEM Frederick Percival Mason, Croydon, England, assignor to Creed and Company Limited, Croydon, Surrey, England Application January 6,1942, Serial No. 425,714

In Great Britain January 24, 1941 7 Claims.

of facsimile scanning head which reduce the ei- 'fect of variation in amplifier gain.

When subject matter comprising marks on a plain background is transmitted over a communication channel by means of a facsimile telegraph system, it is desirable to establish two opposite signaling conditions, one corresponding to the marks (hereinafter called black) and the other to the background (called white), which conditions may be represented by the sending of tone and no tone" respectively. .Now, with normal messages, written in pencil or ink on white or light-coloured paper, it is found that when the pencil or ink is being scanned the amplltude of the tone is not reduced to zero, but to one-third, or perhaps two-thirds, of the amplitude corresponding to white. Consequently, a receiving instrument must beable to discriminate between the full amplitude of the tone corresponding to white and the lower value of the tone corresponding to black. The responding level of the receiver will thus be set at the mid point between these two levels, 1. e., approximately 80-85% of the full amplitude. Under these conditions, it is obvious that should there occur an incidental increase of 15% in the gain of the system, the receiver will record nothing, whereas if the gain decreases by 15% the receiver will record all black. Such incidental variations in gain are to v be expected, particularly in portable apparatus operating from batteries.

The present invention is concerned with the procedure, already known, of subtracting from the signals a constant signal substantially equal in amplitude to the black signal, so that practically no signal is sent during black periods,

while during white periods the dilference only between white and black is transmitted. Thus the transmitted signals consist of tone and no tone, and much larger variations of gain can be tolerated. r

In known methods of carrying this out serious disadvantages are apparent. Thus, in the meth- 06. where the signals at some stage in the ampli- In another known method applicable to chopped light systems, the cancellation is effected at the photo-electric cell stage of the amplifier by producing two out-of-phase light beams directed towards two separate photo-cells, the outputs of which are combined; only one beam is modulated by the signals and the difference between the two photo-cell outputs constitutes the transmitted signal. This method has the disadvantage that two photo-cells and their separate circuits are required instead of one, and variation in sensitivity of the cells wil1 cause inaccurate cancellation. 4

An advantage of the present invention is that, in a chopped light system employing two beams of light from the same lamp, only one photocell is required. 'I'he additional beam of light is directed from the lamp to the chopper and scanning discs in such a way as to cause a second ary beam of light to enter the photo-cell, this beam and the main beam being interrupted outof-phase with each other. The quantity of light in this secondary beam is the same as that in the principal beam when black is being scanned.

Th effect of these two equal out-of-phase beams can be shown graphically to be the same as a steady beam of light entering the cell with a Very small amount of ripple. Since the photocell is reactively coupled to the amplifier, the steady unidirectional potential set up is not communicated to the amplifier; thus, while black is being scanned, no signal is generated. When white is being scanned, the effect of the secondary beam is merely to reduce the signal amplitude. The ratio of black to white signal has thus been increased from to p y 10 It will be seen that, 'with this method, there is no possibility of variations of the amount of cancellation, since one lamp and one photo-electric cell only are used. A simple practical means of carrying the invention into practice is to use a tape masking device, which provides a light reflecting surface of the same width as the aperture itself and adjacent to it.

A description will now be given of the accompanying drawings. In these drawings, Figs. 1 to r 4 show one embodiment of the invention, and Figs. 5 to '7, and Fig. 8 show alternative arrangemen-ts.

Figs. 1 to 4 show the invention as applied to i amount of light.

width equal to the width of the slit itself, 1. e., the width between the edge of the strip and the mask. The total mask aperture, i. e., surface 411 and the surface of the tape 3 exposed in the 8 slit, is brightly illuminated by means of the lamp I and the lens 2. An image of these surfaces is produced on the disc 8 by means of the lens 5. The disc 6 is provided with holes arranged in spiral form, so that, as the disc rotates, succes- 10 sive holes pass across successive elemental portions of the image of the aperture. The size of the holes is arranged so as to be equal to the width of the image of the slit, and the radial distance between adjacent holes is equal to the diameter of the holes (see Fig. 4) Thus, if the disc is in such a position that the imag of the slit falls exactly upon a hole, the image of the mask 4a will fall exactly upon the portion of the disc between adjacent holes. The surface of the mask 4a is treated so that it reflects light with an intensity equal to that reflected from the black portions of the characters marked on the tape. Consequently, if the slit discloses a "black portion of a character, the light reflected from the slit and from the mask will be of equal intensity, and therefore, when a hole in the disc 5 passes successively across the areas corresponding to the slit and the surface, no variation of the light falling onto the photo- 0 electric cell will occur. Since the black portion of the tape and the prepared surface reflect some light, a quantity of light enters the cell. The frame parts 4 of the mask are truly black and of dull surface so as to-reflect a negligible Therefore, when a hole in the disc passes from the frame part of the image to the dully illuminated region corresponding to the image of the prepared surface and "black" tape, the light passing to the cell through this hole suddenly increases from practically zero to a finite value. Since, however, the preceding hole is, at that moment, just leaving the dully illuminated area, the total quantity of light entering the photo-cell is the same ,at every instant. Since the photo-electric cell is reactively coupled with the amplifier, no signal will be generated. If, now, a portion of plain white tape is exposed in the slit, the intensity of the light reflected through the slit will be greater than the intensity of the light reflected from the treated mask surface, and consequently a pulsating beam of light will fall onto the photo-electric cell as the holes in the disc pass the image of the slit, and a signal will be generated in the amplifier.

In the arrangement shown in Figs. 5 to '7, the mask 4 is arranged to expose a plain portion of the tape below the portion on which the char- .acters are written; a concentric row of holes is provided in disc 8 to scan this plain portion; 60 and the lens 5 is arranged to produce an image of the slit on the surface of the disc 6, in such a position that the portion of the slit exposing the characters is scanned by the row of holes arranged in spiral form, and the portion of the slit exposing the tape below the characters is scanned by the concentric row of holes. Immediately in front of the disc 6, and in the path of the beam of light from the portion of tape below the characters, is arranged a filter I2, which may be in the form' of an adjustable film of graduated density, 50 that the amount of light passing through the concentric row of holes may be varied. The concentric row of holes and the spiral row of holes are angularly spaced with reascasce which is in the form of a thin, flat strip, of a spect to one another so that a hole in the concentric row and a hole in the spiral row pass alternately across the image on the slit. The

light passing through the disc is arranged to fall,

greater than the amount of light passing through I the concentric row of holes; thus, a pulsating quantity of light will fall onto the photo-electric cell and a signal will be generated.

It is evident that any suitable means may be employed to control the intensity of the secondary beam in place of the graduated adjustable film l2; for instance, an aperture of adjustable size may be used, but care must be taken that the modulation of the light of the secondary beam is substantially sinusoidal. One method of carrying out this operation is to provide an opaque slide, with one edge at a small angle to itsdirection of. motion, so arranged that, at one extreme limit of its motion, the edge obscures the concentric row of holes, while at the other extreme the holes are exposed. The holes thus travel along the controlling edge, and at midadjustment for example, one-half of the hole is always obscured but the modulation of the light beam remains sinusoidal.

Fig. 8 shows a modification of the arrangement in Figs. 5 to 7. Here the portion of the tape below the characters exposed in the slit is illuminated by a second beam of light, which is controllable in intensity. For this purpose, an image of the filament of the lamp I is projected by means of a second lens system, comprising the lens 8, themirror 9 and the lens Ill. A shutter; II is provided in front of the lens 8 to control the amount of light falling on to the mirror 9. The disc 6 is provided with two rows of holes, as in the arrangement shown in Figs. 5 and 7. The operation of the device is the same as that of the arrangement shown in Fig. 5,

except that the intensity of the beam of light passing through the concentric row of holes is regulated by means of the shutter .ll instead 0 by means of the graduated film l.

'What is claimed is:

l. A facsimile transmitter for transmitting telegraph messages from characters comprising dark area portions written on a running tape comprising a source of scanning light, a mask overlying said tape having a slot extending across said tape through which scanning light from said source may pass, a scanning disc provided with a plurality of spirally arranged openings, means to pick up light from said source reflected back from separate areas in at least two independent beams, one of said beams being reflected from the tape area reserved for said messages and another of said beams being. reflected from an area of constant reflection characteristic and having a constant value corresponding to the dark area portions of the tape, and to project said beams so picked up ontoone surface of said scanning disc, the openings in said scanning disc being disposed so that each opening scans the respective beams one after another, and a photo electric cell disposed on the other side of said scanning disc in the path of rays of light from both of said beams, whereby when dark area portions of tape are being scanned the photo-electric cell receives impulses from both beams of approximately the same low light level.

2. A facsimile transmitter for transmitting telegraph messages from characters comprising dark area portions written on a running tape, comprising a source of scanning light, a mask overlying said tape having a slot extending across said tape through which scanning light from said source may pass, a scanning disc provided with a plurality of spirally arranged openings, means to pick up light from'said source reflected back from separate area portions in at least two independent beams, one beam being reflected from an invariably dark area and having a constant value corresponding to the dark areas of the tape and the other beam being reflected from the writing area of the tape and having a value varying with the light reflected from the light and dark portions of said area and to project said beams onto one surface of said scanning disc in the path of said openings, said openings being disposed so that each scans the respective beams one after another, and a photo electric cell disposed on the other side of said scanning disc in the path of rays of light from both of said beam whereby when dark areas of the tape are being scanned the photoelectric cell receives impulses from both beams of approximately the same low light level and when light area portions of the tape are scanned, the cell receives alternately pulses of high light value and low light value.

3. A facsimile transmitter for transmitting telegraph messages from characters comprising dark area portions written on a running ta comprising a source of scanning light, a mas overlying said tape having a slot extending across said tape through which scanning light from said source may pass, a member of the same size as said slot having the same' light reflecting qualities as the dark areas of the tape and disposed adjacent to said slot in the path of rays of light from said source, a scanning disc provided with a plurality of spirally arranged openings, means comprising optical systems to pick up light from said source reflected from said member and from said slot area, and to project said light onto the surface of said scanning disc in the path of said openings in the form or two independent beams, a light responsive device disposed on the opposite side of said scanning disc, said openings being arranged so that each opening scans one beam after another whereby said light responsive device receives alternately a variable uniform light impulse from said member and a light impulse from said tape, the latter varying with the value of the light reflected from the elemental portions of the writing area of the tape.

4. A facsimile transmitter according to claim 3 ,in which the surface of said mask has less efli- 6. A facsimile transmitter according to claim 2 Q comprising means to pick up light from said light source reflected from a strip of the tape near the edge thereof, of invariable reflection characteristic reflected from an unmarked area of the tape near the edge thereof, and light filtering means disposed in the path of said light to reduce the intensity of said light to substantially that of the light reflected from the dark area portions of the writing.

7. A facsimile transmitter for transmitting telegraph messages from characters comprising dark area portions written on a running tape, comprising a source of light, means for projecting a scanning beam from said source towards said tape, means to pick up light from said source reflected from separate areas in at least two independent beams, one being reflected from an area of invariable reflection characteristic and having a constant value corresponding to the dark area portions of the tape, and the other being reflected from the writing area of the tape and having values varying with the light reflected from the light and dark portions of 'said area, means to project said beams onto a rotating scanning disc provided with a plurality of spirally arranged openings, said beams being beams, and amplifying means reactively coupled with said photo-electric cell.

FREDERICK PERCIVAL MASON. 

